Medical data such as personal medical histories or health information is stored for a long time to keep track of physical conditions that could possibly develop into illness in the future or to allow proper medical treatments. Any leakage of such information causes serious damage because medical information contains personal information.
The risk of information leakage increases as the retention term increases. Examples of known technologies for securely keeping information concealed for a long time include encryption technologies that information—theoretically ensure security.
When an encryption technology that information—theoretically ensures security is used, a communication device requires, for each communicating counterpart, an encryption key having the same length as the plaintext to be communicated. Therefore, the size of the encryption key to be managed by the communication device is increased when there are a large number of communication counterparts or when a long plaintext is to be communicated.